Mature DBA/2J and C57BL/6J mice differ significantly in susceptibility to seizures induced by various physical and chemical stimuli. In general, DBA mice are characterized as relatively "seizure sensitive" and C57 mice are characterized as relatively "seizure resistant". This proposal describes a research project intended to elucidate genomic loci which contribute to the large differences observed between mature C57BL/6J and DBA/2J mice in susceptibility to seizures induced by kainic acid (KA), pentylenetetrazole (PTZ), and maximal electroshock (MES) treatment. It is hypothesized that strain-dependent sensitivity and resistance to aspects of the convulsive response induced by these treatments are mediated by similar genetic elements. In three distinct experiments, B6D2F1 /J (C57BL/6J x DBA/2J, F1 generation) mice will be bred in intercross to produce F2 generations of approximately 500 mice. In each experiment, F2 mice will be studied for seizure phenotype. Behavioral seizure parameters-to be monitored following administration of chemoconvulsants include latency to first seizure (clonus) latency to generalized (tonic-clonic) seizure and number of discrete seizures. In the KA study, induction of and latency to status epileptic us will also be monitored. These parameters will be used to quantify seizure activity in each animal. An overall seizure score will be generated by applying a weighted scale to the quantitative seizure measures. In the third series of experiments, phenotypes will reflect MES threshold for tonic hindlimb extension. The quantitative trait loci (QTL) strategy of "intervals and extremes" will be employed in which F2 progeny exhibiting the 10% most extreme phenotypes (the highest and lowest seizure severity scores or MES thresholds) will be selected for use in genotype experiments. Genotyping will be conducted using PCR-based microsatellite (dinucleotide/simple sequence repeat) DNA markers evenly spaced at 10 cM intervals across the entire mouse genome. Employing a computer program for maximum likelihood methods, MAPMAKER/QTL, genotype data will be analyzed to identify the approximate chromosomal locations of genes contributing to KA and PTZ seizure susceptibility in DBA/2J mice, KA and PTZ seizure resistance in C57BL/6J mice and the difference between the strains in MES threshold. In KA and PTZ studies, separate QTL calculations will be carried out for individual components of the seizure severity score (such as latency to clonus and status epileptic us) as well as for the total seizure score. MES thresholds will be used directly in QTL calculations. Confirmation that similar genetic loci are associated with susceptibility and resistance to specific effects of the described convulsive treatments would indicate the localization of genes important for the regulation of excitability in mouse brain. Identification of these genes could ultimately lead to major advances in understanding and treating human epilepsies.